Read only memory (ROM) is a vital component of modern computers. Beneficially, ROM is a non-volatile memory and, theoretically, can maintain the data it stores indefinitely without power. ROM data is configured in an array of memory cells, each of which has the capability of generating a binary digit. A binary digit is either a logic `1` (high voltage level) or a logic `0` (low voltage level). The basic component of a memory cell is a transistor and hence will be referred to as a memory cell transistor.
As mentioned, read only memory (ROM) is comprised of an array of memory cell transistors, each of which has the capability of generating a binary digit. A ROM circuit is formed by arranging a plurality of memory cell transistors into a matrix (rows and columns). Typically, two sets of 8 transistors are grouped in each row to create two `bytes`, or one `word`, of information for a given row. The gates of all the transistors in each row are connected to a horizontal conductor line referred to as a `word line.` The purpose of the word line is to turn on the gates of all transistors grouped in that word line. Hence, the top word line, coupling sixteen memory cell transistors, is referred to as word line 0, the second word line, linking sixteen different memory cell transistors, is referred to as word line 1, and so on. An x-decoder interprets an input address code to determine the appropriate word-line to which the address refers and then enables that word line.
For a NOR logic arrangement of transistors, the memory cell transistors in a vertical column are either uncoupled from a vertical metal line called a bitline or are coupled in parallel to each other to the bitline. A separate bitline exists for each vertical column of memory cell transistors. For example, sixteen bitlines exist for a word of bits (e.g. 16 binary digits or memory cell transistors in a row). The bitline communicates the actual logic level of the memory cell transistors to other portions of the ROM circuit. A y-decoder interprets an input address code to determine the appropriate bitline to which the address refers and then enables that bitline. With this grid of horizontal and vertical metal lines, each memory cell transistor, and its logic level, can be accessed by enabling the appropriate word line and appropriate bitline to which the desired memory cell transistor is coupled.
Prior Art FIG. 1 illustrates a pair of conventional memory cell transistors within a ROM circuit 100. Memory cell transistor 102 represents a logic level `0` as its drain 103 couples bitline 104 to ground 190. When gate 116 of memory cell transistor 102 is enabled by word line 114, to which it is coupled, its source 118, is coupled to ground 190 via its drain 103. Thus, when bitline 104 is precharged, its voltage level will go to ground because of the coupling just described.
Conversely, memory cell transistor 106 represents a logic level `1` as its drain 107 is uncoupled from bitline 108. When gate 112 of memory cell transistor 106 is enabled by word line 114, to which it is coupled, its source 110, coupled to ground 190, is thereby coupled to ground 190 via its drain 107. Thus, when bitline 108 is precharged, its voltage level will remain at the voltage level of the precharge because of the coupling arrangement just described.
However, the difference in the typical configuration of the memory cell transistors for logic level 0 and logic level 1 state in the prior art creates a problem,. Because memory cell transistor 102 representing a logic level of 0 is coupled to the bitline 104, its transistor body adds capacitance to the bitline in which it is grouped. Conversely, a memory cell transistor 106 representing a logic level of 1 has no portion of the memory cell transistor 106 coupled to bitline 108. Hence it adds no capacitance to bitline 108 in which it is grouped.
Although the difference in capacitance of an individual memory cell transistor may be small, its effect is amplified in at least two circumstances. First, many transistors can be grouped in a single bitline and thus, the additive effect of their capacitance can be substantial. Second, bitlines may be polarized, e.g. have data memory transistors of all one state. Thus, for example, it is possible that all the data memory transistors in one bitline could have 0 logic (increasing the capacitive load on the bitline by the sum of the individual capacitance) while all the data memory transistors in another bitline could have 1 logic (adding no capacitive load to the bitline). With this polarized difference, a significant variation in bitline capacitive loading can occur within the ROM circuit.
Consequently, the variation of the capacitance in bitlines can create a corresponding variation in the precharging and subsequent voltage level and phase from one bitline to the next. As a result of this drawback in the prior art, a need exists in a ROM circuit for a data memory transistor that has an approximately equivalent capacitive loading on the bitline in both the logic 0 state and the logic 1 state.
Prior Art FIG. 2 illustrates a conventional precharge and sensing circuit 200 for conventional ROM memory. Memory cell 202 represents a single memory cell transistor, as discussed above for FIG. 1, as enabled by its corresponding word line 114 and bitline 104. Reference cell 204 has a logic level that will be compared against the logic level of memory cell 202 following a precharge step. PMOS pull-up transistor 206, coupled to power supply voltage 208, supplies a precharge voltage level to memory cell 202 while PMOS pull-up transistor 210, coupled to power supply voltage 212, supplies a voltage level to reference cell 204.
However, NMOS pull-up transistor 214, coupled to power supply 216, and NMOS pull-up transistor 218, coupled to power supply 220, prevent memory cell 202 and reference cell 204 respectively, from achieving a precharge voltage level equivalent to that of power supply 208 and 212. Assuming all power supply voltages 208, 212, 216, and 218 are equivalent, NMOS pull-up transistor 214 and 218 and NMOS bias transistors 222 and 224, for memory cell 202 and reference cell 204 respectively, require a voltage at the source electrode of the transistor to be less than or equal to the gate voltage minus the characteristic threshold voltage of the NMOS transistor.
In other words, memory cell 202 and reference cell 204 cannot be charged to a voltage level equivalent to the voltage level equivalent to that of power supply 208, 212, 216, and 218. Rather, memory cell 202 and reference cell 204 can only be charged to a voltage level equivalent to the voltage level of power supply 208, 212, 216, and 218 minus the threshold voltage level of the NMOS transistors. This prevents the circuit from operating at a power supply voltage level that is less than minimum high logic voltage level plus the threshold voltage. Consequently, a need exists for a precharge circuit with the capability to provide an improved voltage level to memory cell 202 and reference cell 204 when power supply 208, 212, 215 and 220 are at a low-voltage condition.
Additionally, the conventional precharge circuit uses only a single PMOS pull-up transistor 206 and 210 to precharge the memory cell and reference cell respectively. The resultant speed at which the cells are precharged is subsequently limited by this configuration. The limitation subsequently slows down the entire memory retrieval process of ROM data. Consequently, a need exists for a precharge circuit that will precharge the cells in a faster manner.
As a final feature of the conventional precharge circuit, the PMOS pull-up transistor 206 and 210 independently precharge the memory cell and reference cell respectively. Because variation can occur in the individual PMOS transistors and the circuitry leading to the cells, it is possible that the precharge voltage level supplied to the reference cell may be different than the precharge voltage level supplied to the memory cell. Having different precharge voltage levels for the memory and reference cell subsequently consumes a portion of the allowable noise margin in the sensing operation. To improve the noise margin for other variations in the sensing operation, a need exists for reduced variation in the voltage level supplied to the memory cell and reference cell during the precharge operation.
Prior Art FIG. 2 also illustrates sense amplifier 226 used to compare the logic levels of reference cell 204 and memory cell 202 and provide an output equivalent to the logic level of memory cell 202. As mentioned previously, the bitline capacitance can have a significant amount of variation due to the variation in logic levels of memory cell transistors to which it is coupled. Because the reference cell, e.g. dummy bitline, configuration does not change, it thereby has a constant capacitance. Consequently, when the voltage level of the reference cell with a constant capacitance is compared to the voltage level of a bitlines, whose capacitance could be radically different from any other bitline, a significant variation in voltage amplitude and phase can occur. As a result, the sense amplifier that measures the differences in voltage amplitude and phase between the memory cell and the reference cell may misinterpret a phase shift as a logic level different from what the memory cell actually is. Hence, the entire sensing operation may be prone to error or may require a longer amount of time to achieve a steady state conditions for all the potential variations in capacitance of a bitline. In view of these limitations, a need exists for a dummy bitline configuration with approximately equivalent capacitance as a bitline such that sensing speed and accuracy may be enhanced.
The conventional sensing operation of a memory cell only references one memory cell 202 as indicated in Prior Art FIG. 2. Although not indicated in Prior Art FIG. 2, the balance of the memory cells in a ROM circuit having a plurality of memory cells and bitlines are left floating with whatever residual charges they may have in their respective bitline. Interestingly, the bitlines contain a metal conductor line that must be separated by insulators from adjacent bitlines in order to prevent a short circuit. Hence, an unintended capacitive structure evolves between adjacent bitlines. Unfortunately, residual charges left on disabled bitlines surrounding the enabled bitline to be sensed will influence the amount of precharge accepted on the desired bitline and will influence the phase and amplitude of the signal from the desired bitline during the subsequent sensing operation. This influence, commonly referred to as `cross-talk,` consumes some of the noise margin in the sensing operation and may lead to errors in output and overall degraded performance of the ROM circuit. Because of the potential drawbacks of the conventional operation, a need exists for reducing cross-talk between adjacent bitlines and reducing its negative side-effects.
In summary, a need exists for a data memory transistor, within a ROM circuit, that has more consistent capacitive loading on the bitline in both the logic 0 state and the logic 1 state. Furthermore, a need exists for a precharge circuit, within a ROM circuit, with the capability to provide an acceptable voltage level to the memory cell and the reference cell at low-voltage power supply conditions. At the same time, a need exists for the precharge circuit to precharge the cells in a faster manner. Besides the aforementioned needs, a further need exists for reduced noise in the voltage level supplied to the memory cell and reference cell during the precharge operation. Still another need exists for a dummy bitline configuration with approximately equivalent capacitance as a bitline such that sensing speed and accuracy may be enhanced. Finally, a need exists for reducing cross-talk between adjacent bitlines and reducing its negative side-effects.